1. Field of the Invention
The invention relates to a resolution enhancer circuit for providing increased resolution for an analog to digital (A/D) converter. More specifically, the invention relates to a signal processing system which performs a "windowing" function to position the current value of a large-range input voltage to a value that can be digitized within the operating range of the A/D converter.
2. Description of the Related Art
When designing a signal processing system which includes an A/D converter, the A/D converter must be selected to provide adequate resolution for all signals processed by the system. That is, the A/D converter must operate over the entire voltage range of an input signal, leading to the use of costly high resolution A/D converters to meet design requirements.
For example, in systems which use an 8-bit microcontroller with an 8-bit A/D converter integrated into the microcontroller, the microcontroller may have to be replaced by an external 12, 16 or 32 bit A/D converter to meet performance requirements. The result is increased cost, circuit area, power consumption and decreased circuit reliability.
U.S. Pat. No. 4,901,078, issued to Goyal, discusses a "window detector" used with a recirculating A/D converter. In the Goyal device, an input signal is sent through the A/D twice in order to obtain a high resolution digital output. U.S. Pat. No. 5,296,856, issued to Mantong, discusses another type of "window detector" A/D converter, in which the time between two successive out-of-limit signals is used to compute a high resolution digital output.
With these conventional devices, there must either be performed multiple runs of the same analog input value, or there is needed a timing mechanism to determine the digital signal based on two successive out-of-limit signals, leading to high complexity and increased cost.
Therefore, it is desirable to have an inexpensive circuit which can provide increased resolution for an n-bit A/D converter, thereby alleviating the need to use an A/D converter that is greater than n bits to achieve the desired resolution.